copper fineness coins
SILVER ALLOYS. - Pure silver is too soft to make durable coins or vessels combining lightness with stability of form. This defect can be cured by alloying it with a little copper. All ordinary "silver" articles consist of such alloys. The proportion of silver in these (their "fineness") is habitually stated in parts of real silver per 1000 parts of alloy. In Great Britain all silver coins are made of "standard silver," the fineness of which, by legal definition, is 925. The toleration is 4 units (of pure silver in 1000 of alloy), i.e., a specimen passes as long as its fineness lies between 925 and 921 (compare MINT, vol. xvi. p. 483). As regards silver-plate the "Hall" in London refuses to stamp any poorer alloy. In Germany and in the United States all silver coins, in France and Austria the major silver coins, are of the fineness 900, with a toleration of 3 units. The minor coins of Austria are of the fineness 375 to 520; in France all silver coins under one franc contain 835 of silver, 93 of copper, and 72 of zinc in 1000 parts. The fineness prescribed by law or custom for " silver " articles is 950 or 800 (f 5) iu France, 750 in North Germany, 812-5 in South Germany, and 820 in Austria. All these alloys at least arc liable to " liquation," which means that, although they are perfectly homogeneous in the crucible, they freeze into layers of not absolutely the same composition. According to Leval, passing from the skin to the core of an ingot of 900 per mille silver the difference may amount to 3 units. Of all the alloys tried by that chemist only that composed according to the formula Ag3Cu2, corresponding to 719 per mille of silver, remained perfectly homogeneous on freezing. He therefore recommends this alloy for coinage; unfortunately, however, any silver-copper alloy which contains less than about 750 per mille of noble metal tarnishes very perceptibly in the air. British standard silver is quite free of this defect, but it is inconveniently soft, far softer than the "900" alloy.
The extent to which the properties of silver are modified by addition of copper depends on the fineness of the alloy produced. The addition of even three parts of copper to one of silver does not quite obliterate the whiteness of the noble metal. According to Kaniarsch the relative abrasion suffered by silver coins of the degrees of fineness named is as follows :- Abrasion 1 2.3 3'9 9.5 The same observer established the following relation between fineness p and specific gravity in coins containing from 375 to 875 of silver per 1000 : - sp. gr. =0.001647p+8.833.
The fusing points of all copper-silver alloys lies below that of pure copper; that of British standard silver is lower than even that of pure silver. For the alloys of silver with other metals than copper, see GOLD, PLATINUM, and NICKEL. The present writer has introduced an alloy of 91 of silver, 7 of gold, and 2 of nickel as a material far superior, on account of its higher rigidity, to fine silver for the making of alkali-proof vessels.
"Oxidized" silver is ordinary cupriferous silver superficially modified by immersion into sulphide of sodium solution (which produces a dark film of sulphide), or otherwise.
Silvering. - For the production of a silver coating on a base-metallic object we have chiefly two methods. One of these is to dissolve silver in mercury and to apply this amalgam to the (carefully cleaned) surface of the object by means of a brush. The mercury then is driven away by heat, when a coherent film of silver remains, which adheres very firmly, is quite continuous, and needs not be thick to stand polishing and other surface treatment. This very old method is to tIna day the best for producing a strong coating, but it is dangerous to the health of the workmen, expensive, and troublesome, and has been almost superseded by the modern process of electroplating (see ELECTED-METALLURGY, vol. viii. p. 116). Objects made of iron or steel must first be coated over with copper, and then treated as if they consisted of that metal.
For Glass-Silvering, see limanon, vol. xvi. p. 500.
Inscriptions on linen, consisting of black metallic silver and consequently proof against all ordinary processes of washing, can be produced by using suitably-contrived silver solutions as inks. A mere solution of nitrate of silver (1 to 8 of water)' will do, if the surface to which it is applied has been prepared by impregnation with a solution of 6 parts of soda crystals and 17 of gum arabic in 30 of water, and subsequent ironing. The ink must be applied with a quill or gold pen (compare vol. xiii. p. 81).